This invention relates to an apparatus and a method of selectively providing tone and announcement message PCM codes to different channels of a telephonic network and, more particularly, to such a tone and announcement generating apparatus and method in which different preselected sequences of tones and announcement codes are selectivley provided to, and selectively received from, a plurality of time division multiplexed channels associated with the generating apparatus.
In telephonic switching networks, there is a need for automatic communication between the network control and the individual telephonic transceiving units. This communication takes two different forms of messages: tones and announcements. Tones are single, multiple, or alternating frequencies, and are continuous or intermittent. Familiar examples of such tones include the busy tone, ring back tone, dial tone, and touch tones. The announcements are voice communications such as "Your call cannot be completed," and "Calls are now being taken at . . . ". These tone and announcement messages have been stored in memory in the form of binary, pulse code modulated, or PCM, signals which may be referred to generically as message codes. The message codes comprise a series of binary words representing the amplitudes of a corresponding series of samples or segments of the tone or announcement. When these message codes are converted to analog form, they produce an audio signal convertible to sound by means of an electromagnetic speaker.
In a time division multiplexing system, it is necessary to selectively provide these message codes to a plurality of different telephonic transceiving units respectively associated with a plurality of time slots or channels which compose a multi-channel time division multiplexing frame. In the past, a separate ROM (Read Only Memory) has been provided for each multifrequency tone code, and the sample words have been read out simultaneously from all the ROM's, one word per frame per tone being read out and available during essentially the entire frame. To switch a tone to a desired destination in a switching network, the output of the tone of that ROM was sent into the switching network in the network time slot, or channel, associated with the desired destination requiring such a tone.
In U.S. Pat. No. 4,205,203 of Mehta et al., issued May 27, 1980, to the predecessor in title of the assignee of this application, instead of the tone codes being made available during essentially the entire time interval, each multi-channel cycle, or frame, and switching each tone code sample into a channel time slot, or channel, the individual tone codes samples are made available only during the relatively short time slot, or channel, of each frame when the tone code is needed. Command signals received asynchronously from a central control unit results in the outputing of any desired one of many tones in any desired one of the several time slots within the recurring cycles, or frames. In addition, a predetermined selection of certain "fixed tones" is provided. These are always outputed in preassigned time slots of every frame, while other tones are selectively switched into the remainder of the time slots or channels. Operational sound samples held in a main memory at successive address locations are selected by applying, during one time slot of one cycle, address signals read out of a certain location of a next-sample read/write memory and, thereafter, inserting into that certain location, new signals which are incremented or decremented by one in relation to those previously existing, and writing out the new address signals during the same time slot of the ensuing cycle.
While this tone generator operated satisfactorily, there were some disadvantages. No provision was made for handling any message codes other than tone codes. There was no capability of providing announcement message codes for voice communications. Flexibility was limited because all message codes of a given sequence had to be stored in consecutive address locations because only identification codes were stored. The remaining addresses had to be derived from the address for the first sample of the tone sequence.
Limitations were also present. Since the tone codes were preferably stored in read only memories, there was no ability of reprogramming the tone code memory via the telephonic network, and of course, there was no message recording capability. Moreover, the tone codes could not be down loaded into the tone code memory from a central tone code memory, and thus, a change of tones required a change read only memories. In addition, periodic tone signals were generated by storing only the tone portion and then periodically gating the PCM codes of the tone in accordance with a preselected duty cycle memory. Accordingly, the stored PCM signals had to be given special treatment as compared to the PCM codes of other tones which also reduced system flexibility.